1. Field of the Invention
The invention relates to a power steering system for use in automotive vehicles and, more particularly, to improvements in a power steering system including a rotary control valve adapted for providing positive steering stability.
2. Description of the Prior Art
Power steering systems containing a control valve are well-known in the art which include a torsion bar connecting an input stub shaft and an output worm shaft to provide relative angular displacement therebetween thereby permitting the control valve to switch introduction of pressurized fluid from a fluid pump between two working chambers defined at the opposite sides of the power piston. Such power steering systems can provide smooth steering performance with a less amount of torque required for the vehicle operator to apply to the steering wheel by the aid of fluid pressure exerting on the power piston. It has been found, however, that problems occur if the required steering torque is determined only by the spring constant of the torsion bar as down in the prior art. Where a torsion bar is designed to have an excessively less spring constant for the purpose of further input torque reduction, it will readily be twisted to start powr steering performance even when the road load is low and no power steering performance is required such as during high speed running; whereas it will have its size increased too much to be assembled in the prepared space and the power steering responsibility will be reduced with the torsion bar designed to have an excessively high spring constant for the purpose of providing steering reaction or "feel" similar to that of normal mechanical steering systems.
The control valve incorporated in such power steering systems normally comprises a spool valve or a rotary valve, and the latter has been found superior to the former in simplified construction and reliable performance. Such a rotary valve mainly comprises an inner sleeve rotatable by one of the input and output members and formed on its outer periphery with axially extending equally spaced slots, and a concentric outer sleeve mounted around the inner sleeve in sliding and sealing contact therewith for rotation with the other thereof and formed on its inner periphery with axially extending equally spaced slots which are associated with the first described slots to switch the flow of pressurized fluid from a fluid pump between the working chambers. In such conventional rotary valves, however, it is the normal practice to supply high pressure fluid to the outside of the outer sleeve and hence introduce it through the slots into the predetermined fluid passage, while low pressure fluid is drained along the inside of the inner sleeve so that the inner sleeve is subject to axial compression forces due to pressure difference between the opposite sides thereof which cause a strain of the inner sleeve resulting in spoiled sealing performance between the inner and outer sleeves and reduced switching performance.